fix: --state-values-set panic: value of type interface {} is not assignable to type string (#680)

Probably since #647 helmfile has been unable to merge nested maps in environment values if they were loaded from files. This fixes it.

The relevant test is also enhanced so that no further regression like this happens.

Fixes #677

pkg/tmpl/file_renderer.go

@@ -46,6 +46,8 @@ func (r *FileRenderer) RenderTemplateFileToBuffer(file string) (*bytes.Buffer, e
 	return r.RenderTemplateContentToBuffer(content)
 }
 
+// RenderToBytes loads the content of the file.
+// If its extension is `gotmpl` it treats the content as a go template and renders it.
 func (r *FileRenderer) RenderToBytes(path string) ([]byte, error) {
 	var yamlBytes []byte
 	splits := strings.Split(path, ".")